All water-soluble phosphate salts such as soluble fertilizers are derived from phosphoric acid. Phosphoric acid is produced commercially by either a ‘wet’ or a thermal process. Wet digestion of phosphate rock is the most common process. Thermal processing is energy intensive and therefore expensive. For that reason, quantities of acid produced thermally are much smaller and mainly used for production of industrial phosphates.
Phosphoric acid for fertilizer production is almost solely based on wet digestion of rock phosphate. The process is mainly based on dissolution of apatite with sulfuric acid. After dissolution of the rock, calcium sulfate (gypsum) and phosphoric acid are separated by filtration. To produce merchant-grade phosphoric acid, high acid concentrations are required and water is evaporated. Calcium sulfate exists in a number of different crystal forms depending on the prevailing conditions such as temperature, phosphorus concentration in the slurry, and level of free sulfate. Calcium sulfate is either precipitated as di-hydrate (CaSO4.2H2O) or as hemi-hydrate (CaSO4.½H2O). Phosphoric acid produced through this process is characterized by a relatively low purity.
For deriving ammonium phosphate salts, merchant-grade phosphoric acid, having a concentration of about 54% P2O5, is neutralized with ammonia to form either mono-ammonium phosphate (MAP) or di-ammonium phosphate (DAP) by controlling the ammonia-to-phosphoric acid mole ratio during the neutralization process. Ammonia is used in liquid or gaseous form. Liquid anhydrous ammonia is usually preferred since surplus heat from other systems in necessary for vaporizing liquid ammonia into a gaseous form.
The neutralization of merchant-grade phosphoric acid with ammonia is usually performed in several stages using several reaction vessels. The mole ratio of ammonia to phosphoric acid in the pre-reactor/s is normally held at a level which gives the maximum solubility for the slurry (between 1.4 and 1.45 for production of DAP and usually less than 1 for production of MAP). For operation control, the ammonia to phosphoric acid mole ratio is determined by monitoring the pH of the slurry. Excess heat of reaction is removed from the pre-neutralizer/s by adding water to the reactor/s. Evaporation of the water cools the slurry. As the mole ratio of ammonia to phosphoric acid is increased over 1, un-reacted ammonia escapes from the reactor and the gaseous vapors released must be scrubbed with an acid.
The slurry from the pre-neutralization reactor/s which usually contain between 16 to 23% water is usually fed into an ammoniator-granulator to complete the addition of ammonia for the desired product. Completion of the neutralization and additional evaporation of water results in solid particles being formed. It is necessary to recover the un-reacted ammonia from the gaseous vapors by scrubbing with an acid. Thereafter, the solid ammonium phosphates are usually dried in a separate reactor to reduce moisture content. Loss of ammonia from the dryer is usually recovered by scrubbing with acid. The solid ammonium phosphates are normally cooled by passing air through a cooling reactor.
For several applications such as fertigation, i.e. the application of water-soluble fertilizers in the irrigation water, and foliar fertilization, i.e. spraying fertilizers on leaves, there is a need for fully-soluble ammonium phosphates to avoid clogging of the fertigation equipment by non-dissolved solids. Wet-process phosphoric acid contains a substantial amount of impurities such as iron, aluminum, calcium, magnesium, cadmium, etc. which form water-insoluble solids upon neutralization with ammonia and thus fertilizer-grade ammonium phosphates are not completely water-soluble. Therefore, fully-soluble P fertilizers for fertigation purposes must be specially produced from purified phosphoric acid which means additional processing.
The current technology for phosphoric acid purification is based on extraction of impure wet-process phosphoric acid into an organic solvent (ketones, tri-alkyl phosphates, alcohols, etc.) followed by back extraction with water forming a pure phosphoric acid but with a lower concentration, which is thereafter concentrated by water evaporation. Purified phosphoric acid is thereafter neutralized with ammonia forming fully-soluble ammonium phosphate products according to the procedure described above.
The disadvantages of the state-of-the art technologies for production of ammonium phosphates are numerous. The phosphoric acid as produced from the gypsum filter, in a dihydrate process, is not suitable for direct manufacture of ammonium phosphate salts. The acid must be further concentrated by water evaporation to a suitable phosphoric acid concentration (usually about 54% P2O5). Normally, concentration of phosphoric acid is done in three stages. The acid from the filter (28% P2O5) is evaporated to 40% P2O5 in a single stage vacuum evaporator. The acid is then clarified to remove precipitated solids and the clarified acid is then concentrated to 54% P2O5 in two stages. The inter-stage concentration is about 48% P2O5. The 54% P2O5 acid is used for ammonium phosphate production according to the procedure described above.
To concentrate acids through evaporation is a very energy-intensive process. The amount of steam required for concentrating phosphoric acid usually varies between 2.5-5 tons of steam per ton of phosphorus, depending on production conditions. If the phosphoric acid is purified by solvent extraction the energy demand is about 7 tons steam per ton of phosphorus. The energy demand for concentration of phosphoric acid is a major production cost. Expensive equipment such as steam distribution systems, evaporators, effluent gas scrubbers, condensation systems, cooling water systems, liquid effluent treatment systems and acid storage facilities are necessary for production of merchant-grade phosphoric acid. About 50 tons of cooling water is required in order to condense one ton of vapor. In a barometric condenser the vapor is directly contacted by the water and as a result impurities in the vapor contaminate the cooling water which results in large quantities of contaminated effluents. Furthermore, additional equipment is needed for the neutralization of phosphoric acid with ammonia in several stages, drying, cooling and scrubbing of ammonia from gaseous vapors. Production of ammonium phosphate of technical quality requires additional processing steps as described above.
U.S. Pat. No. 3,894,143 describes a process for obtaining crystallized ammonium phosphate from wet-process phosphoric acid and ammonia. The process consists of a) forming a mixture of aqueous phosphoric acid and acetone in which all components are miscible with water, b) precipitating impurities by addition of ammonia and separating the precipitated impurities to form a purified mixture, c) contacting the purified mixture with ammonia to produce ammonium phosphate crystals and a supernatant liquid, and d) separating the ammonium phosphate crystals from the supernatant liquid and distilling the supernatant to separate the acetone for recycling. The disadvantages of this method include distillation of large quantities of acetone, limited yield of ammonium phosphates, and production of large quantities of dilute aqueous ammonium phosphate effluents. A further main limitation of this procedure is the insufficient selectivity. Water miscible solvents are approaching the properties of water which results in that cationic and anionic contaminants are co-extracted to a large extent.
U.S. Pat. Nos. 3,975,178 and 4,236,911 are similar to U.S. Pat. No. 3,894,143 and consist of forming a mixture of aqueous phosphoric acid and acetone or methanol in which all components are miscible with water followed by addition of ammonia in order to precipitate impurities and thereafter ammonium phosphate. The disadvantages are similar to that reported for U.S. Pat. No. 3,894,143.
U.S. Pat. No. 4,132,540 describes a process for removing solvent from the raffinate of extracted phosphoric acid. Residual solvent is removed from the raffinate by addition of ammonium or alkali or alkaline earth metal cations in an atomic ratio to phosphorus of between 0.1:1 and 0.6:1.
U.S. Pat. No. 4,311,681 describes a process for separation of impurities such as silica and organic impurities from an organic solvent by washing with an aqueous alkali orthophosphate solution sufficient to maintain the pH of the solvent-aqueous mixture at from about 9.5 to about 12.5
U.S. Pat. No. 4,678,650 describes a process for production of an aqueous alkali phosphate solution by mixing an aqueous phase containing an alkali compound with an organic phase containing phosphoric acid in a volume ratio larger than 1:1, and thereafter separating the resulting aqueous alkali phosphate solution from the organic phase.
U.S. Pat. No. 4,751,066 describes a process for the alkaline stripping of wet process phosphoric acid from a water immiscible organic solvent in order to produce a sodium phosphate solution.
In the published international patent application WO 2008/115121, a method and an arrangement for phosphorus recovery are disclosed. Phosphorus ions are extracted from solutions by adsorbing phosphorus ions in a scavenger and by releasing the phosphorus ions into an eluate during regeneration of the scavenger. The regeneration is performed by ammonia. Phosphate anions are precipitated in form of tri-ammonium phosphate upon introduction of excess amounts of ammonia. The ammonia remaining in solution after the precipitation of tri-ammonium phosphate is reused for regenerating the scavenger. Unfortunately, tri-ammonium phosphate is unstable at ambient temperature and atmospheric pressure resulting in the decomposition of the crystal accompanied with release of ammonia which requires further processing into stable forms of ammonium phosphate. Tri-ammonium phosphate is not suitable for direct use in agriculture.
GB 636,035 discloses improvements of processes of producing diammonium phosphate. Crystals of mono-ammonium phosphate are introduced into a solution of diammonium phosphate in a reactor and anhydrous ammonia is fed into the reactor. Diammonium phosphate crystals are collected at the chamber bottom.
In the U.S. Pat. No. 3,415,619, a process for making ammonium phosphate is disclosed. Water-soluble ammonium phosphate is achieved by extracting a substantially iron-free aqueous phosphoric acid, derived from the reaction of calcium phosphate-containing ore and a strong mineral acid, into a water-immiscible extractant, separating the phosphoric acid-laden extractant from the residual aqueous phase, removing the calcium impurities therefrom, contacting the phosphoric acid-laden extractant with anhydrous ammonia at a temperature of between about 20 and 90° C., and separating solid, water-soluble ammonium phosphate from the extractant. The solid ammonium phosphate is indicated to be washed with low boiling hydrocarbon solvents to remove organic extractant adhesive thereto. The process according to this disclosure has several drawbacks. In experiments performed by the applicant of the present invention, it was concluded that it is very difficult to produce di-ammonium phosphate according to the presented ideas even with a large excess of ammonia. The main difficulty of this approach is thus that large amounts of solvent remain adhering to the precipitated ammonium phosphate crystals and this loss of expensive solvent would be economically unacceptable. Removal of adhering solvent by distillation is difficult since the boiling point for solvents such as tributyl phosphate (289° C.) exceeds the melting point for mono-ammonium phosphate (190° C.). Washing the adhering solvent with another hydrocarbon solvent was proved to be incomplete, and considerable amounts of extractant remain even after extensive washing. Furthermore, separation of extractant and hydrocarbon solvent become a complex and expensive task due to the need for distillation.
The published international patent application WO 2010/138045 describes a process comprising addition of ammonia to a phosphorus-loaded water immiscible liquid phase in order to precipitate ammonium phosphates. The precipitated ammonium phosphates are washed with saturated aqueous solution of ammonium phosphate and the washed crystals are dried. Residual scavenger washed form the crystals is separated by a phase separation of the scavenger and the saturated aqueous solution of ammonium phosphate and the separated residual scavenger is reused for further adsorbing of phosphorus to be reused for further extraction. The washing liquid depleted from residual scavenger is reused for further washing of the crystals. A disadvantage is the need for three phase separation.
U.S. Pat. No. 3,518,071 describes a process for production of nitrophosphate fertilizer and ammonium-nitrate calcium-carbonate fertilizers. The process consist of a) digesting phosphate rock with nitric acid to produce a solution of calcium nitrate, phosphoric acid and nitric acid, b) extracting phosphoric acid and nitric acid from the leach solution with amyl alcohol, c) re-extracting phosphoric acid and nitric acid with a concentrated solution of mainly ammonium nitrate containing some ammonium phosphate, with a N/P molar ratio of about 95:1, d) evaporating the strip solution to form a slurry of crystallized ammonium phosphate and ammonium nitrate, e) separating the crystals from the liquor, f) recycling at least a portion of the liquor to be used in step c, g) treating the raffinate from step b with ammonia and carbon dioxide to form ammonium nitrate solution and precipitated calcium carbonate.
The main purpose of U.S. Pat. No. 3,518,071 is to use liquid-liquid extraction for separation of calcium nitrate instead of separating calcium nitrate by precipitation at low temperatures which is the common practice at the nitrophosphate process. There are a number of disadvantages with this method. The final product is generally a mixture of ammonium phosphate and ammonium nitrate and not well-defined mono-ammonium phosphate or di-ammonium phosphate. Furthermore, co-extraction of nitric acid results in precipitation of calcium phosphates which render the liquid-liquid extraction process non-operational. This limits the yield of extracted phosphoric acid. Moreover, a high concentration of ammonium nitrate is required in the solution used for stripping in order to decrease the amount of ammonium phosphate that remains dissolved in the ammonium nitrate liquor. Also, the method requires processing of ammonium nitrate into a final product by water evaporation. Furthermore, the phosphate product contains ammonium nitrate via mother liquor adhering to the precipitate. Moreover, crystallization is obtained by exceeding the solubility of dissolved salts by evaporation of water which requires equipment such as evaporators, steam distribution systems, etc., as well as, an energy source. Finally, amyl alcohol and other solvents suitable for this process have a high water solubility which makes the process complex as the solvent has to be recovered from the aqueous streams by extraction and distillation.
The patent GB 1,238,188 describes a process for production of ammonium phosphates by digesting phosphate rock with nitric acid. The process consist of a) treating the leach solution with a solvent composed of aliphatic alcohols containing 4-6 carbon atoms to extract nitric acid and phosphoric acid, b) purifying the organic extractant by contacting the extractant with a solution composed of mainly ammonium nitrate, c) ammoniating the purified extractant to form a precipitate of ammonium phosphate, a light phase containing the solvent, and a heavy aqueous phase containing ammonium nitrate and ammonium phosphate, d) separating the light phase by decanting and recycling the decanted light phase to the extraction step, e) separating the ammonium phosphate precipitate from the heavy aqueous phase, f) washing the separated ammonium phosphate with water, g) the washing water of ammonium phosphate and part of the heavy aqueous phase containing ammonium nitrate and ammonium phosphate is recycled to the extractant purification step, h) the aqueous solution from the purification step is being recycled into the extraction step, i) the raffinate from the extraction step and part of the heavy aqueous phase containing ammonium nitrate and ammonium phosphate is treated by distillation to recover the solvent from the aqueous phase, and j) treating the distilled aqueous phase with ammonia and carbon dioxide to form ammonium nitrate solution and precipitated calcium carbonate.
Similar to U.S. Pat. No. 3,518,071, there are a number of disadvantages with the method according to the patent GB 1,238,188. Co-extraction of nitric acid results in precipitation of calcium phosphates which limits the yield of extracted phosphoric acid from the leach solution. Furthermore, a high concentration of ammonium nitrate is required during neutralization with ammonia in order to decrease the amount of ammonium phosphate that remains dissolved in the ammonium nitrate liquor. Also, the method requires processing of ammonium nitrate into a final product by water evaporation. Furthermore, the precipitated ammonium phosphate contains ammonium nitrate via mother liquor adhering to the precipitate and washing of the adhering ammonium nitrate with water results in substantial dissolution of the precipitated ammonium phosphate. This requires acidification of the wash water with nitric acid (to convert dissolved ammonium phosphate into ammonium nitrate and phosphoric acid) and re-extraction of phosphoric acid from the wash water which is costly. Finally, solvents suitable for this process have a high water solubility which makes the process complex as the solvent has to be recovered from the aqueous stream by distillation.
U.S. Pat. No. 4,112,118 describes a process for stripping phosphoric acid from an organic solvent with a basic compound of ammonia, sodium or potassium being anhydrous or with water in an amount of up to 10 moles or with a solid dihydrogen phosphate salt, to give a liquid phase mixture comprising an organic solvent phase substantially free of phosphoric acid and an aqueous phase comprising dissolved phosphoric acid and dissolved dihydrogen phosphate of the base. One main disadvantage of the process is that the stripping process requires that the organic solvent is loaded with a highly concentrated phosphoric acid solution of >60% weight. The process thus requires concentration of phosphoric acid by water evaporation, equipment such as evaporators, steam distribution systems, etc., as well as, an energy source. Another main disadvantage of the process is that the product is an aqueous solution which requires further treatment.
Russian patent 424849 describes a method for production of mono-ammonium phosphate by using a solvent composed of tributyl phosphate in a kerosene diluent. The method comprises contacting the tributyl phosphate solvent, which is loaded with phosphoric acid, with a solution of di-ammonium phosphate at an organic to aqueous phase ratio of about 3:1 to form a depleted solvent and mono-ammonium phosphate crystals in a solution of mono-ammonium phosphate. It is further mentioned that the mother liquor can be separated and treated with ammonia to produce a di-ammonium phosphate solution which can be recycled.
It was, however, found by many experiments performed by the applicant of the present invention that stripping a solvent composed of tributyl phosphate in kerosene with a di-ammonium phosphate solution according to the procedure described in Russian patent 424849 results in severe emulsion formation when the stripped solvent is re-contacted with phosphoric acid for further phosphate extraction. The formation of emulsion makes the process non-operational in industrial applications where efficient recirculating of solvent is required.